Maskless lithography

ABSTRACT

The present invention provides a method for maskless lithography. A plurality of individually addressable and rotatable micromirrors together comprise a two-dimensional array of micromirrors. Each micromirror in the two-dimensional array can be envisioned as an individually addressable element in the picture that comprises the circuit pattern desired. As each micromirror is addressed it rotates so as to reflect light from a light source onto a portion of the photoresist coated wafer thereby forming a pixel within the circuit pattern. By electronically addressing a two-dimensional array of these micromirrors in the proper sequence a circuit pattern that is comprised of these individual pixels can be constructed on a microchip. The reflecting surface of the micromirror is configured in such a way as to overcome coherence and diffraction effects in order to produce circuit elements having straight sides.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under contract no.DE-AC04-94AL85000 awarded by the U.S. Department of Energy to SandiaCorporation. The Government has certain rights in the invention.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to co-pending provisional applicationSer. No. 60/020,090, Filed Jun. 19, 1996, Entitled PROGRAMMABLE MASK FORSCANNING PROJECTION LITHOGRAPHY, from which benefit is claimed under 35U.S.C. §119(e).

BACKGROUND OF THE INVENTION

This invention pertains generally to a method for forming patterns on asemiconductor wafer and to an apparatus to practice the method and, moreparticularly, to maskless projection lithography.

At the present time, the patterns on semiconductor wafers, whichrepresent electronic components and their interconnections, are producedby "writing" the pattern from a mask onto a photoresist covered wafer bya process such as projection lithography. The conventional projectionlithographic process employed for producing electronic circuits onsemiconductor wafers is similar to exposing a film negative ontophotographic paper except that the transferred image is reduced(typically by from 4× to 10×) by a camera rather than enlarged, therebymaking the circuit elements smaller. This process is illustrated inFIG. 1. Light from a light source 110 shines onto a mask 120 containinga circuit pattern. During the process, mask 120 is caused to move in onedirection. The light that penetrates the mask pattern, representing thecircuit pattern desired to be reproduced, is focused onto a photoresistcoated wafer 130 by camera 140 which forms a focused image of the maskdemagnified (reduced) by a factor of typically between 4× and 10×. Thewafer 130 is moving in a direction opposite to that of mask 120 suchthat the image of the mask features are stationary on the wafer.

While effective, this process has numerous drawbacks associatedprincipally with the masks such as the cost of fabricating masks, thetime required to produce the sets of masks needed to fabricatemicrochips, diffraction effects resulting from light from light source110 being diffracted from opaque portions of the mask, registrationerrors during mask alignment for multilevel patterns, color centersformed in the mask substrate, defects in the mask or the presence ofdust particles on the mask that are rendered as imperfections in thecircuit pattern, the necessity for periodic mask cleaning and thedeterioration of the mask that follows therefrom. These drawbacks areparticularly pronounced during the process of producing prototypemicrochips. Any minor error or flaw in the circuit design layout or anychange in the circuit design can require the fabrication of a full setof masks; an expensive and time consuming process. The aforementioneddrawbacks are particularly noticeable when producing small lots ofmicrochips or specialty microchips. What is needed is a method foreliminating the use of a mask in fabricating circuit patterns onsemiconductor wafers.

Responsive to these needs the present invention provides a method forproducing circuit patterns on semiconductor wafers without the need fora mask.

SUMMARY OF THE INVENTION

The present invention overcomes deficiencies associated withconventional methods of fabricating microchips by replacing the maskused in a conventional scanning projection lithography system with atwo-dimensional array of micromirrors that are used to produce a circuitpattern on a semiconductor wafer. The present invention furthereliminates problems of mask alignment during the intricate adjustmentsof the wafer stage and multi-level micro-circuitry, the effect ofdefects in the mask and provides for the use of extreme ultravioletlight sources.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form part ofthe specification, illustrate the present invention and, together withthe description, explain the invention. In the drawings like elementsare referred to by like numbers.

FIG. 1 illustrates prior art scanning lithography system.

FIG. 2 illustrates an embodiment of the present invention.

FIG. 3 shows the principle of operation of the invention.

FIG. 4 shows an interleaved micromirror array.

FIG. 5A illustrates micromirror orientation at time t=t₀.

FIG. 5B illustrates micromirror orientation at time t=t₀ +Δt.

FIG. 5C illustrates micromirror orientation at time t=t₀ +2Δt.

FIG. 6 illustrates an optimized micromirror shape.

FIG. 7 shows a calculated line shape printed by an optimized micromirrorshape.

FIG. 8 shows a calculated via hole printed by an optimized micro mirror.

DESCRIPTION OF THE INVENTION

The present invention overcomes the drawbacks associated withconventional methods of fabricating microchips by replacing the mask 120used in a projection lithography system with a plurality of rotatablemicromirrors that can be arranged in a two-dimensional array, such asillustrated in FIG. 2. As used hereinafter, the term micromirror refersto a reflecting surface 225, disposed on a substrate 226, whoseprincipal dimension is ≈μm, typically ≈20 μm or less. By replacing themask used in conventional projection lithography, the inventiondescribed herein greatly reduces the cost of fabricating small lots ofmicrochips and further, speeds up the process of prototyping newmicrochip designs. Here a plurality of individually addressable androtatable micromirrors 220, that together comprise a two-dimensionalarray 210, reflect light from a light source 230 through camera 140 thatforms a focused but demagnified (reduced) image of the micromirror, ontophotoresist coated wafer 130.

Each micromirror 220 in array 210 can be envisioned as an individuallyaddressable element in the picture that comprises the circuit patterndesired. As each micromirror is addressed it rotates so as to reflectlight from light source 230 onto a portion of the photoresist coatedwafer thereby forming a pixel within the circuit pattern. Byelectronically addressing a two-dimensional array of these micromirrorsin the proper sequence a circuit pattern that is comprised of theseindividual pixel elements can be constructed on a microchip.

The present invention also provides significant advantage in theproduction of binary optical elements (BOE). High efficiency BOEs aremade by contact printing and typically require four masks to create 16phase levels. Thus, a complete set of fabrication steps must be carriedout for each level, i.e., spin on the resist, expose the resist andprocess, while maintaining the mask aligned to the substrate throughoutthe processing sequence. However, by using the method of masklesslithography disclosed herein, it is possible to create the differentphase levels by means of variable level exposure, i.e., the step heightis now a function of the exposure level.

FIG. 3 generally sets forth the method of operation of a two-dimensionalarray of micromirrors such as 210. An electronically generated maskpattern is caused to move across the face of array 210, in much the sameway words scroll across an electronic signboard, by causing thosemicromirrors which are supposed to be dark, such as illustrated bymicromirror 310, to be rotated slightly such that light from a lightsource reflected from micromirror 310 is not intercepted by camera 140(i.e., micromirror 310 is in the "off" position). On the other hand,those micromirrors, such as 320, that are intended to reflect light froma light source onto photoresist coated wafer 130, thereby forming apixel, are rotated such that the light reflected from micromirror 320 isintercepted by camera 140 and projected onto photoresist coated wafer130 (i.e., micromirror 320 is in the "on" position). As is the case inconventional scanning lithography, the image moving across array 210projected by camera 140 will move in synchrony with wafer 130.

In one embodiment of the present invention, the array of micromirrors isilluminated with a light source comprising a pulsed laser, particularlyan excimer laser and preferably a KrF, F or ArF excimer laser. A highrepetition rate light source is preferred in order to give a writingspeed of about 5 mm/s (i.e., 20 kHz). In a second embodiment, the arrayof micromirrors is illuminated with a pulsed or mechanically choppedsource having a wide wavelength spectrum, particularly a discharge lampand preferably a high pressure arc lamp. In a third embodiment, at leasttwo arrays of micromirrors can be interleaved, as illustrated in FIG. 4,in order to produce a greater number of pixels on the photoresist coatedwafer with the same reticle dimension, thereby eliminating the need forstep and repeat processing. Interleaved arrays also provide forminimizing boundary errors. Further, increasing the array size byinterleaving micromirror arrays, as described herein, also provides forincreasing resolution, thereby improving the quality of the imageprojected onto the photoresist coated wafer. As shown in FIG. 4, atwo-dimensional array of pixels can be produced by interleaving twoseparate arrays of micromirrors A and B horizontally such that columnsof A micromirrors alternate with columns of B micromirrors. By way ofexample, columns marked A can be written by the upper array ofmicromirrors U and columns marked B can be written by lower array ofmicromirrors L.

In a further embodiment, rows of micromirrors can be interleavedvertically in time rather than in space, by loading a new image andpulsing the light source every time the wafer moves one row. FIG. 5shows one column of mirrors and the image data for that column. Here,odd and even rows of the image are multiplexed together in time. At timet=t₀, FIG. 5A, the odd rows in the image will be printed by dataresiding in data registers 510. These data control the orientation ofmicromirrors (causing them to be tilted either into a reflecting (+)"on" position 320 or non-reflecting (o) "off" position 310). During thisstep, data describing printing in the even rows is stored in "hidden"registers 511. At time t=t₀ +Δt, FIG. 5B, the data controlling theorientation of micromirrors has moved down one register such that datathat was stored in data registers 511 are now in register 510 andcontrol the micromirrors, thus even rows are printed; data that was instored in data registers 510 are now "hidden" in registers 511. At timet=t₀ +2Δt, FIG. 5C, the data controlling the micromirrors has moved downanother position and once again the odd rows are printed. In this waydata point P, which is to be converted into one pixel on the wafer, hastranslated from mirror 520 (FIG. 5A) to mirror 530 (FIG. 5C).

An image generation means has been described using a two-dimensionalarray of micromirrors, and interleaving in both time and space. Forsimplicity, one of the axes of the mirror array has been described asparallel to the scan direction in this description. Other orientationsare also contemplated; for instance, the array axes could be oriented at45° to the scan direction. For this example, the size of themicromirrors and the spacing between pixels in the scan direction andperpendicular to it are both smaller by the factor √2.

It is contemplated that data handling circuitry such as that used in acharge coupled device (CCD) can be used to move the data that controlsthe orientation of micromirrors from one register to another. Here athree step voltage field moves data, which exists as an electric charge,down one register at a time. The data (charge) moves down one row perpulse of the light source. Simultaneously, the photoresist coated wafermoves the width of one pixel per pulse of light source 230. Othermethods of moving data controlling micromirror movement will be obviousto those skilled in the art. The forgoing description is intended to beillustrative of the present invention and is not to be construed as alimitation or restriction thereon.

Conventional micromirrors used in spatial light modulators are built on17-micron centers. If these arrays were used to write a 25-mm wide chipwith 0.5-micron features, the demagnification of the camera would haveto be 34:1. Thus the array (or arrays) in the mask plane would have tobe 850 mm long. This would require 1 meter class optics in the camerawhich would be excessively expensive. The size of the object plane, andtherefore the size of the camera optics can be reduced by reducing themicromirror center-to-center spacing or interleaving two arrays as willbe described next.

The horizontal and vertical features (the pattern) produced on aphotoresist coated semiconductor wafer by the present invention willpreferably be composed of a plurality of pixels or small squares eachprinted on the photoresist coating. For example, a conductor in thecircuit can be a row of pixels. In order that features printed on thesemiconductor will have straight smooth edges, it is necessary that eachpixel be as square as possible. However, diffraction effects can operateto defeat that requirement. If the light intensity at the corners of theimage of a reflecting surface of a micromirror is significantly reducedthe pixel produced on the photoresist will be rounded and consequently,the pattern produced on the microchip will have a scalloped edge.

Pronounced variations in light intensity at the boundaries, caused bycoherence effects, can cause short circuits to be formed in theresulting circuit pattern. It is possible to create an optimum shape foreach pixel image in the presence of diffraction by modifying the shapeof the micromirror reflecting surface while optimizing the temporal andspatial characteristics of the illumination.

It has been found that the shape of the reflecting surface can beoptimized so smooth-sided lines can be printed, where the narrowestlines are either a single row or a single column of pixels. Therefore, arow of pixel images that are incoherent to one-another should add suchthat the full-width, half-maximum (FWHM) (or some other intensitycontour) of the line is of constant width. This FWHM of a line of imagescan be defined by two straight, parallel lines that are spaced one pixelwidth apart.

If the pixels generated by a row of unoptimized square micromirrors areprinted, they will create a narrow, scalloped line if the width of theline is near the Critical Dimension (CD), or printing limit, of thelithography system. If the FWHM of the line to be printed is supposed tobe equal to the geometrically calculated pixel width, the square mirrorshave to be 5-10% larger than the desired line width. The lines will nowbe wide enough at the center of each pixel but they will still bescalloped and 10-15% too narrow at the pixel intersections. The shape ofthe reflecting surface can be sculpted to correct this; for example,various projections or "ears" can be added to the corners of thereflecting surface as shown in FIG. 6. FIG. 6 is a reflecting surfaceshape that will print 0.25 μm wide lines using 248 nm light (numericalaperture=0.7 and quadrapole illumination). FIG. 7 is a computedintensity profile for the line that would be printed on a wafer usingthe mirror geometry of FIG. 6. Referring now to FIG. 7, in thecalculations point "A" receives intensity contributions from pixels 1and 2 and point "B" has intensity contributions from pixels 1, 2 and 3.FIG. 8 is the intensity profile at the wafer plane of one pixel. Itcould print a via hole on the wafer.

It is well known in the art of lithographic mask design that featuressuch as the projections or "ears" shown in FIG. 6 can be added to amask, thus changing the intensity profile of the image, which improvesthe shape of the printed features on the wafer. For example, a maskmight be altered so it prints more-or-less square via holes. The samewell-known design technique can be used to optimize the shapes of thereflecting surfaces of the micromirrors of the present invention; exceptthe design criteria are different and unique. Here the micromirrors aredesigned such that when a row of pixels generated by these optimizedmicromirrors are printed, each one being incoherent with respect to itsneighbors, the resulting image will have straight sides (at someintensity contour).

There are other ancillary features, well known in the art, that can beadded to a mask to optimize the printed image. Examples includephase-reversed features (commonly known as phase masks) and small and/orattenuated ancillary features whose images are not intense enough toprint, but contribute coherently to the desired image. Squaring of theimage of the micromirror into the entrance pupil of camera 140 can befurther improved with the use of "quadrapole" or annular shapes andoff-axis illumination as will be obvious to those skilled in the art.The description of reflecting features that can be added to thereflecting surfaces of the micromirrors to reduce diffraction effectsand alternative illumination means is intended to be illustrative of thepresent invention and is not to be construed as a limitation orrestriction thereon.

The inventors have discovered that interleaving the micromirrors in bothspace and time permits smaller optics, printing of smaller features onthe wafer, and improved line width control. The inventors have furtherdiscovered that by reducing the size of the reflecting surfaces on eachsubstrate such that the reflecting surfaces are no longer juxtaposed,coherence effects which cause the light intensity at the boundariesbetween pixels to be unpredictable (typically ranging from zero to one)can be substantially eliminated, thereby improving the quality of theimage printed on the photoresist coated semiconductor wafer. In apreferred geometry, the reflecting surface on each tiltable substratecan be roughly half the size of the micromirror center-to-center spacing(as measured in the scan direction). This separation between the pixelsthat are being printed at any one instant in time makes their imagesnearly incoherent with respect to one-another. Furthermore, the adjacentpixels that are printed at different times will also be incoherent withthose mentioned above. Hence the printed image will be the incoherentsum of all the pixels printed and thus, smooth-sided lines can beprinted.

In the present invention reflecting surfaces smaller than themicromirror substrates can be made by several processes known to thoseskilled in the art. For example, a non-reflective coating can bedeposited onto the reflecting surface of the conventional micromirrorand then a mask designed to limit the size of the reflecting area can bedeposited on the reflecting surface of the micromirror. A newmicromirror having the proper dimension and shape, such as 220 (FIG. 2)or FIG. 6, can be formed by etching away part of the non-reflectivecoating. These smaller size micromirrors can then be interleaved both inspace and time as set forth hereinabove.

The demand for smaller critical dimensions in advanced computer chips iscontinuing to spur improvements in projection lithography. Presently,deep ultraviolet lithography systems, operating at 248 nm and producing0.25 μm features, are commercially available and 193 nm lithographysystems, capable of producing features in the 0.18 μm range, are underdevelopment. In order to produce smaller features it is necessary tooperate at even shorter wavelengths. By utilizing extreme ultraviolet(EUV) radiation in the range of 4.5-15 nm it is possible to producefeatures smaller than 0.18 μm. The resolution and therefore, the minimumfeature size that can be obtained with EUV is a factor of 2-6 timesbetter than with the present deep-UV or 193 nm lithography. The presentinvention contemplates the use of EUV radiation as a light source.However, the use of EUV radiation to produce patterns smaller than 0.18μm on microchips poses unique problems.

Moreover, EUV radiation is absorbed by ordinary mirror surfaces such asglass or silicon. Therefore, in applications wherein EUVL radiation isused special mirror surfaces must be provided. In one embodiment of thepresent invention all the reflective micromirror surfaces needed toreflect EUV radiation are coated with precisely matched multilayer Braggreflective coatings comprising periodic alternating layers of molybdenumand silicon or molybdenum and beryllium having bilayer periods equal toapproximately half of the reflected wavelength at normal incidence.Moreover, these multilayer coatings can be deposited on the micromirrorelements with graded multilayer periods to maintain the wavelength ofpeak reflectance as the angle of incidence changes across the figure ofeach micromirror element. Further, the shape of the reflecting parts ofthe micromirror can be defined by depositing a non-reflective maskingmaterial onto the portions of the substrate that should not reflect.

SEQUENCE LISTING

Not Applicable.

We claim:
 1. A mask for scanning projection lithography, comprising:a) atwo-dimensional array of individually addressable micromirrors, eachsaid micromirror comprising a plurality of moveable substrates having areflecting surface, said reflecting surfaces for reflecting light from alight source into an imaging means entrance pupil, said reflectingsurfaces modified by coating said surfaces with a non-reflecting coatingand etching away only a portion of said non-reflective coating, therebyproviding said reflecting surfaces with features for substantiallyeliminating coherence and diffraction effects in said reflected light,said features comprising:i) a reflecting surface having a shapeconsisting essentially of an octagon having regular trapezoidalprojections laying at every other side of said octagon and wherein thewider base of each of said trapezoidal projections is contiguous withsaid octagon, ii) a reflecting surface having a size which is aboutone-half of a center-to-center distance between adjacent micromirrors;and iii) a reflecting surface have an edge which includes a plurality ofphase reversed features and small light attenuating features along saidedge; said light reflected by each of said micromirrors comprising apixel having a light distribution profile, said reflected light imagedby said imaging means at a distant focal plane, said imaged lightcomprising rows or columns of pixels arranged so that adjacent pixelsadds constructively to providing continuous, substantially smooth,straight regions of light and dark lines, said lines having a minimumwidth which is a single row or a single column of said pixels; b) meansfor individually moving each of said micromirrors in saidtwo-dimensional array of micromirrors; c) means for controlling themovement and orientation of each of said micromirrors such that saidreflected light is directed either into or away from said imaging meansentrance pupil; and d) sequencing means in communication with saidcontrolling means, said sequencing means for causing each of saidmicromirrors to move in proper sequence so as to scan a predeterminedcoordinate pattern across said two dimensional array of micromirrors, ina scan direction, thereby causing said pattern to be reproduced at saidfocal plane, said sequencing means including temporally interleavingrows of said micromirrors where said rows are parallel to said scandirection.
 2. The apparatus of claim 1, wherein said light sourcecomprise a pulsed light source.
 3. The apparatus of claim 2, wherein thepulsed light source is a pulsed laser.
 4. The apparatus of claim 2,wherein said light source includes a discharge source.
 5. The apparatusof claim 4, wherein the discharge source is configured to produce alight pulse.
 6. The apparatus of claim 4, wherein the source is a pulsedplasma.
 7. The apparatus of claim 1, wherein said reflected light isimaged into said imaging means entrance pupil as an annular orquadrapole shape or as off-axis illumination.
 8. The apparatus of claim1, wherein the two-dimensional array of micromirrors is produced byoptically interleaving at least two separate arrays of micromirrors. 9.The apparatus of claim 1, wherein the two dimensional array ofmicromirrors comprise at least two sets of micromirrors, wherein the twosets are separate and offset from one another and temporallyinterleaved.
 10. The apparatus of claim 1, wherein the size of thereflecting surface is configured such that coherence effects betweennearby pixels are substantially eliminated.
 11. The apparatus of claim1, wherein the reflecting surfaces of the micromirrors are coated with aprecisely matched multilayer Bragg reflection.
 12. The apparatus ofclaim 11, wherein the multilayer Bragg coating is Mo/Si or Mo/Be havingbilayer periods equal to about one-half of said reflected lightwavelength at normal incidence.
 13. The apparatus of claim 11, whereinthe multilayer coating is deposited with graded multilayer periods tomaintain the wavelength of peak reflectance as the angle of incidencechanges across each micromirror.